For the effective separation of small particles such as microorganisms or macromolecules from a liquid, it is necessary to prevent clogging of the filter surface by suitable measures. The application of a thick layer of filtering aids on a rotating filter surface in revolving filters is well-known for this purpose; a thin layer is removed from the thick layer together with the separated particles during each rotation.
In another known process, definite quantities of kieselguhr are added continuously in measured amounts as a filtering aid to the liquid to be filtered, settle together with the filtered particles on the filter surface and, by means of their porosity and their grain size, prevent clogging of the filter surface for a limited time. From time to time, the built-up filter cake must be removed and a new filtration cycle commenced by depositing a new filter layer.
In common prior art filters, which may comprise vertical, horizontal and/or cylindrical services, there is no appreciable liquid flow parallel to the filter surface during filtration. In such common systems, static pressure increases in value up to several atmospheres during operation and very high static pressures are required in the first instance, to drive liquid through the filter surface. Static pressure is defined as the intrinsic liquid pressure acting perpendicular to boundary walls of the container therefore. The significance in this case being the membrane filter surface.
Fermented liquids, especially vinegar, wine, cider have been filtered until now by classic filtration methods only using filter surfaces (e.g. fine screens) precoated with Kieselgur or asbestos and pressing the liquid through this layer by means of static pressure between about 5 and 70 psi without any significant movement of the liquid parallel to the filter surface. Filter aid must continuously be added in order to keep the filter cake open as long as possible. After a certain time of filtration, while pressure increases, filter rate decreases to the extent that the filter cake must be washed out after interruption of the filtration and the procedure of precoating and filtration begins again. The different steps can be controlled by hand or automatically. The filter cake gives waste problems. The step of precoating is critical. Continuous dosing of filter aid and control of filter pressure are necessary. Filtration can be done during working hours only.
Apart from small laboratory models state of the art apparatus have been in use in technical scale for two basic purposes
(In the following materials reference is made to reference R being an article by Porter et al, "Membrane Ultrafiltration", appearing in Chemical Technology, January, 1971, pages 59-63, and also to reference S, being an article by D. Michaels, "New Separation Techniques For the Chemical Process Industries", Chemical Engineering Progress, December, 1968, pages 31-43.)
a. "Ultrafiltration for hydraulic pressure activated separation of solutions into their individual components by passage through synthetic semipermeable membranes" and PA0 b. "Reverse Osmosis for separation of low-molecular weight solutes, e.g., salts, sugars, simple acids, etc. from their solvent (usually water)." (Page 32, lines 6 and 24, of Reference S, cited below) PA0 "boundary layer of substantially more concentrated solution (relative to the midchannel concentration) adjacent to the membrane" (Ref. S, page 35 last four lines). PA0 "If the solution is sufficiently concentrated and the solute of relatively low molecular weight, the high solute osmotic pressure in the boundary layer will reduce the effective pressure for ultrafiltration, thereby reducing the ultrafiltration flux. If the solution contains a high-molecularweight solute or colloid, solute accumulation at the membrane surface produces a layer of finite, and frequently large, hydraulic flow resistance." (Ref. S, page 36, lines 30 - 43). PA0 "This accumulation leads to formation of a "slime" or "cake" on the membrane, which increasingly impedes solvent flow through the membrane until convective transport of solute towards the membrane surface equals the rate of back diffusive transport away from the membrane." (Ref. R, page 59, lines 21 - 26, right column). PA0 "High-molecular-weight solutes, such as the proteins and colloidal dispersions when concentrated beyond a certain point form solid or thixotropic gels" (Ref. R, Page 60, lines 1 - 4). PA0 a. Reverse Osmosis: "For relatively low permeability membranes, e.g. 10 - 20 gfd at 500 - 1500 psi, and solutes such as sodium chloride, polarization moduli can be held to reasonably small values (1.5) in even rather large membrane bounded conduits ( 1 in. tubes, 1/4 in. slits) and with moderate fluid velocities in turbulent flow." (Ref. S, page 36, last two lines -- page 37, line 7) PA0 b. "Ultrafiltration of high-molecular-weight solute containing solutions of colloidal dispersions, presents polarization problems of yet a different dimension." (Ref. S, page 37, line 41). PA0 "Because of the extremely low diffusion coefficients of macromolecules and colloids in solution, the minimization of polarization (and realization of high ultrafiltration rates) is far more critical than in the case of microsolutes. In turbulent flow systems, astronomically high fluid velocities, large pressure drops and necessarily high fluid recirculation rates are required to achieve ultrafiltration rates much over 10 - 15 gfd. On the other hand, by operating in the laminar flow regime, at quite low fluid velocities in thin channels ( 5 - 20 mils), fluxes as high as 70 - 75 gfd can be realized with very low energy input and little to no fluid recirculation." (Ref. S, page 37, last four lines -- page 38, line 9). PA0 "These systems cannot, of course, accept process streams which contain significant amounts of coarse suspended matter. Such streams must either be prefiltered settled or centrifuged to remove large particles prior to ultrafiltration, or processed in wide conduit, turbulent flow systems at much lower ultra-filtration rates." (Ref. S, page 38, lines 16 - 25)
"Ultrafiltration is the term applied to the separation of relatively high molecular weight solutes (e.g., proteins, natural gums, polymers, other complex organic compounds) and colloidally dispersed substances such as clays, pigments, minerals, latex particles, microorganisms, etc. from their solvents (usually water). In these systems the osmotic pressure of the solute is usually negligible and plays no important role in the separation process. PA1 In Reverse Osmosis, the driving pressure for efficient separation must exceed the osmotic pressure of the solute in the solution." (Ref. S, page 31, lines 29 - 46), PA1 "Therefore for Reverse Osmosis pressures between 500 and 2000 psi are often required". (Page 57, line 24, Reference R, cited below)
But also Ultrafiltration of high-molecular-weight solutes and or colloidally dispersed substances are two different methods. In Ultrafiltration of solutes concentration polarization produces a
Therefore to avoid concentration polarization during Ultrafiltration of solutes, the task is to remove a higher concentrated liquid layer from adjacent the membrane.
Unless otherwise indicated references herein to % are weight per cent and references to lengths, height, diameters weights and cross-section ratios of suspended particles are to worst cases of such parameters for design purposes.
To avoid concentration polarization during Ultrafiltration of colloidally dispersed solids, the task is to avoid a layer of solids on the membrane.
This is a problem which has been solved by the present application. In contrast the prior art uses the following approaches:
In other words, there is no special polarization problem in the reverse osmosis process in spite of the high pressures used in such process.
German Pat. No. 1,020,000 discloses a process for separating a solution of diverse molecular composition by ultrafiltration at high pressure by means of semipermeable membranes, in which the solution is forced along at the membrane so that it is maintained in the immediate vicinity of the membrane in a continual turbulent motion. The membrane should have a rough profile for this, at least in the longitudinal cross-section. The patent also calls for assuring homogeneity of the liquid using turbulence generated in the liquid. Solid matter is not present in the liquid. Pressures of, for example, 10 atmospheres are provided for.
German Pat. No. 1,154,439 describes a process for the extraction of solid matter in enriched and purified form, wherein a suspension is guided past a filter, impermeable to the solid matter, in the circulation circuit above a supply vessel, with a change of the transport direction executed at definite time intervals, whereby eddy currents are formed and the solid matter should be retained in front of the filter. Extraction of the solid matter is subsequently accomplished by flushing back.
The above described processes have the disadvantage that they cannot be used for continuous filtration of liquids that contain microorganisms, macromolecules or solid matter since the proposed measures are insufficient to completely prevent clogging of the particles on the filter.
It is the object of the present invention to provide a process in which these disadvantages are eliminated which is suited both for extraction of large quantities of filtrate and also of the solid matter to be filtered in enriched form.
It is a further object of the invention to provide distinctly advantageous apparatus for the process.